and cattle. The syndrome is named after the Cuban physician M. Che´diak and the Japanese pediatrician O. Higashi who described the condition in 1952 and 1954, respectively. See Aleutian mink. cheetah See Acinonyx jubatus. chelating agent a compound made up of hetero- cyclic rings that forms a chelate with metal ions. Heme (q.v.) is an example of an iron chelate. The porphyrin ring in chlorophyll (q.v.) forms a magne- sium chelate.
chelation the holding of a metal ion by two or more atoms of a chelating agent. Chelicerata a subphylum of arthropods containing the species that have no antennae and possess pin- cerlike chelicerae as the first pair of appendages. See classification. chemical bonds See disulfide linkage, electrostatic bond, glycosidic bonds, high-energy bond, hydrogen bond, hydrophobic bonding, ionic bond, peptide bond, phosphodiester, salt linkage, van der Waals forces. chemical elements listed alphabetically by their symbols. The biologically important elements are shown in boldface type. See periodic table.
chemiosmotic theory the concept that hydrogen ions are pumped across the inner mitochondrial membrane, or across the thylakoid membrane of chloroplasts, as a result of electrons passing through the electron transport chain (q.v.). The electrochem- ical gradient that results is the proton motive force (pmf). ATP synthase harnesses the pmf to make ATP See Appendix C, 1961, Mitchell; adenosine phosphate, mitochondrial proton transport. chemoautotrophy See autotroph, methanogens. chemokines a large family of structurally homolo- gous cytokines (q.v.), 8 to 10 kilodaltons (kDa) in size.
The name “chemokine” is a contraction of “che- motactic cytokine.” Chemokines share the ability to stimulate leukocytic movement (chemokinesis) and directed movement (chemotaxis), especially of in- flammatory cells to damaged or infected sites. Exam- ples of chemokines include interleukin-8 (IL-8) that attracts neutrophils, basophils, and eosinophils, and monocyte chemotactic protein-1 (MCP-1) that acts specifically only on monocytes. Chemokines are pro- duced by several types of cells, including activated mononuclear phagocytes, tissue cells (endothelium, fibroblasts), and megakaryocytes (which give rise to platelets that contain stored chemokine). chemolithoautotroph an autotroph that gets its energy from oxidation of inorganic substances in the
absence of light. Many hyperthermophiles use inor- ganic electron donors and acceptors in their energy metabolism and obtain their carbon from CO2. See lithotroph, Methanococcus jannaschii. chemostat an apparatus allowing the continuous cultivation of bacterial populations in a constant, competitive environment. Bacteria compete for a limiting nutrient in the medium. The medium is slowly added to the culture, and used medium plus bacteria are siphoned off at the same rate.
The con- centration of the limiting nutrient in the fresh me- dium determines the density of the steady-state population, and the rate at which the medium is pumped into the chemostat determines the bacterial growth rate. In chemostat experiments, environ- mental variables can be changed, one by one, to as- certain how these affect natural selection, or the en- vironment can be held constant and the differential fitness of two mutations can be evaluated. chemotaxis the attraction or repulsion of cells or organisms toward or away from a diffusing sub- stance.
Also known as chemotropism. chemotherapy the treatment of a disease with drugs of known chemical composition that are spe- cifically toxic to the etiological microorganisms and do not harm the host. The term was coined by Paul Ehrlich, who also gave such drugs the nickname magic bullets. See Salvarsan. chemotrophs organisms whose energy is the result of endogenous, light-independent chemical reactions.
A chemotroph that obtains its energy by metaboliz- ing inorganic substrates is called a chemolithotroph, whereas one that metabolizes organic substrates is called a chemoorganotroph. Contrast with prototrophs. See autotrophs. chiasma (plural chiasmata) the cytological mani- festation of crossing over; the cross-shaped points of junction between nonsister chromatids first seen in diplotene tetrads. See Appendix C, 1909, Janssens; 1929, Darlington; crossing over, meiosis, recombina- tion nodules.
chiasma interference the more frequent (in the case of negative chiasma interference) or less fre- quent (in the case of positive interference) occur- rence of more than one chiasma in a bivalent seg- ment than expected by chance. chiasmata See chiasma. chiasmatype theory the theory that crossing over between nonsister chromatids results in chiasma for- mation. chicken See Gallus domesticus. chimera an individual composed of a mixture of genetically different cells. In plant chimeras, the mixture may involve cells of identical nuclear geno- types, but containing different plastid types.
In more recent definitions, chimeras are distinguished from mosaics (q.v.) by requiring that the genetically dif- ferent cells of chimeras be derived from genetically different zygotes. See also aggregation chimera, het- erologous chimera, mericlinal chimera, periclinal chi- mera, radiation chimera. chimpanzee See Pan. Chinchilla lanigera a rodent native to the Andes mountains of South America. It is bred on commer- cial ranches for its pelt, and many coat color mutants are available. Its haploid chromosome number is 32. CHIP-28 an abbreviation for CHannel-forming In- tegral Protein of 28 kDa relative molecular mass.
CHIP-28 was purified from the plasma membranes of human erythrocytes and later shown to form channels permeable to water. It was renamed aquap- orin1 (AQP1). chiral descriptive of any molecules that exist in two mirror-image versions (enantiomers, q.v.). Chironomus a genus of delicate, primitive, gnat- like flies that spend their larval stage in ponds and slow streams. Nuclei from various larval tissues con- tain giant polytene chromosomes.
The salivary gland chromosomes of C. thummi and C. tentans have been mapped, and the transcription processes going on in certain Balbiani rings (q.v.) have been studied extensively. See Appendix C, 1881, Balbiani; 1952, Beermann; 1960, Clever and Karlson; chromosomal puff. chi sequence an octomeric sequence in E.
coli DNA, occurring about once every 10 kilobases, act- ing as a “hotspot” for RecA-mediated genetic recom- bination. chi-square ( 2) test a statistical procedure that en- ables the investigator to determine how closely an experimentally obtained set of values fits a given theoretical expectation.
The relation between χ2 and probability is presented graphically on page 74. See Appendix C, 1900, Pearson; degrees of freedom. chi structure a structure resembling the Greek let- ter χ, formed by cleaving a dimeric circle with a re- striction endonuclease that cuts each DNA circle only once. The parental monomeric duplex DNA molecules remain connected by a region of hetero- duplex DNA at the point where crossing over oc-
Chi square ( 2) test and Student t test probability chart